Perishable beverage and food products are often placed into containers at elevated temperatures. In a conventional hot-fill process, the liquid or flowable product is charged into a container at elevated temperatures, such as 180 to 190 degrees F., under approximately atmospheric pressure. Because a cap hermetically seals the product within the container while the product is at the hot-filling temperature, hot-fill plastic containers are subject to negative internal pressure (that is, relative to ambient pressure) upon cooling and contraction of the products and any entrapped air in the head-space.
It has been a goal of conventional hot-fill container design to form stiff cylindrical portions (in transverse cross section) that maintain a cylindrical shape upon cooling. Thus, conventional hot-fill containers include designated flexing portions—vacuum panels—that deform when subject to typical hot-fill negative internal pressures. The inward deflection of the vacuum panels tends to equalize the pressure differential between the interior and exterior of the container—that is, absorb vacuum—so as to enhance the ability of the cylindrical sections to maintain an attractive shape, to enhance the ease of labeling, or like commercial appeal. Some container designs are symmetric about a longitudinal centerline and designed with stiffeners to maintain the intended cylindrical shape while the vacuum panels deflect. For example, U.S. Pat. Nos. 5,178,289, 5,092,475, and 5,054,632 teach stiffening portions or ribs to increase hoop stiffness and eliminate bulges while integral vacuum panels collapse inwardly. U.S. Pat. No. 4,863,046 is designed to provide volumetric shrinkage of less than one percent in hot-fill applications.
Other containers include a pair of vacuum panels, each of which has an indentation or grip portion enabling the container to be gripped between a user's thumb and fingers. For example, U.S. Pat. No. 5,141,120 teaches a bottle having a hinge continuously surrounding a vacuum panel, which includes indentations for gripping. In response to cooling of the container contents, the hinge enables the entire vacuum panel to collapse inwardly. U.S. Pat. No. 5,141,121 similarly teaches a bottle having an outward bulge that inverts in response to cooling of the container contents. Each of the patents referred to herein by patent number is incorporated by reference in its entirety.
It has been observed that for some containers undergoing vacuum conditions, inward deflection of portions of the container, such as panels, causes regions circumferentially spaced apart from the panels to deflect outwardly. For example, some containers having opposing handgrips, which may tend to deflect inwardly upon vacuum conditions, have label panels that may deflect outwardly under vacuum conditions.
Also, some containers are subject to creasing. For example, edges of a flex panel may locally bulge outwardly after hot-filling, which is unattractive.